The invention relates to a change-speed gearbox.
In a known change-speed gear mechanism of the type stated at the outset (FR 1.003.128) it is necessary that both the step interval between the gears should always be the same and that the relationship between the transmission ratios of the gear wheel stages of the synchronizing mechanism should be equal to the constant step interval between the gears. As a result, synchronization can only be performed if the gears are selected sequentiallyxe2x80x94in other words not if selection involves skipping one or more gearsxe2x80x94and only with the main clutch disengaged. In one embodiment of this known change-speed gearbox, the synchronizing mechanism is controlled in such a way by a control device operating as a function of the engine speed that the gear wheel stage having the transmission ratio greater than 1 is used when a lower limiting value of the engine speed is reached and the other gear wheel stage is used when an upper limiting value of the engine speed is reached.
In a known change-speed gearbox of the layshaft type (WO 91/13271 A1), which does not belong to the generic type, gear wheel clutches without synchronizing elements for the gear wheel stages associated with the gears and a central synchronizing mechanism are provided. Gear changes are performed in the manner of a power shift with the main clutch engaged, by way of the synchronizing mechanism, which is used in the form of a gear wheel stage with a transmission ratio equal to or less than the transmission ratio of the highest forward gear or in the form of an infinitely variable friction mechanism with an endless wrap-round means that connects adjustable conical drive pulleys. In this known change-speed gearbox, the synchronizing mechanism can only be effective in the case of gear changes with the engine in traction mode - conditions for synchronization in the case of gear changes with the engine in over drive mode are not discussed in this publication.
In another known change-speed gearbox (EP 0 202 800 B1) of a different type, a two-stage rear-mounted transmission is arranged after a main layshaft-type gearbox with five forward gears and one reverse gear in the power transmission path, the said transmission doubling the number of gears of the main gearbox. Arranged between the two coaxial layshafts of the main gearbox and the rear-mounted transmission in the power transmission path is a synchronizing gear mechanism in the form of an epicyclic mechanism, the planet carrier of which is in continuous drive connection with the output shaft of the main gearbox via the layshaft of the rear-mounted transmissionxe2x80x94the outer central wheel of which can be braked by means of a friction brake, which is engaged in the xe2x80x9cdownshiftxe2x80x9d synchronization statexe2x80x94and the inner central wheel of which is both continuously connected to the layshaft of the main gearbox and can be braked by means of another friction brake, which is engaged in the xe2x80x9cupshiftxe2x80x9d synchronization state. In this known change-speed gearbox, the main clutch is disengaged during each gear change, with the result that power shifts are not possible, and the synchronizing gear mechanism""s brake, which acts directly on the layshaft of the main gearbox, would dissipate valuable driving energy in the event of a hypothetical gear change performed with the main clutch engaged.
The object underlying the invention is essentially to be regarded as enabling gear changes, including those which involve skipping several gears, i.e. freely selectable gear changes with a power-shift character, without unnecessary dissipation of driving energy.
In the change-speed gearbox according to the invention, the power flow is limited to only an insignificant extent during the gear change because the engine remains connected continuously to the output shaft owing to the retention of the engaged state of the main clutch, and a transmission state with a definitive transmission ratio is set in the synchronizing gear mechanism during synchronization, even in the case of an upshift. Compared with a conventional gear change mechanism with a synchronizing process which involves a complete interruption of torque transmission, in all cases an improvement in the direction of a power shift character with the change-speed gearbox according to the invention is achieved.
In the change-speed gearbox according to the invention, the outlay in terms of construction is advantageously reduced by using both gear wheel stages of the synchronizing gear mechanism both for synchronizing and to form a gear ratio.
In the change-speed gearbox according to the invention, the fact that that gear wheel stage of the synchronizing mechanism which has the transmission ratio greater than 1 operates independently of the main clutch means that freely selectable gear changes with a power shift character are possible even with the engine in overdrive mode.
In embodiments of the change-speed gearbox the use of gear wheel stages in the form of a respective stationary transmission for the synchronizing gear mechanism has proven advantageous.
In other embodiments of the change-speed gearbox according to the invention, it was possible to keep the installation space in the directions of the transmission shafts particularly low by using an epicyclic mechanism for one gear wheel stage of the synchronizing gear mechanism in accordance with.
Other embodiments of the change speed gearbox relate to an advantageous way of incorporating the epicyclic mechanism forming one gear wheel stage of the synchronizing gear mechanism into the change-speed gearbox according to the invention.
Other embodiments of the change speed gearbox relate to the configuration of the epicyclic mechanism used as one gear wheel stage of the synchronizing gear mechanism in the change-speed gearbox according to the invention, part-ratios of two of these embodiments being used to form respective gear ratios.
In one embodiment of the change speed gearbox, also provided to form gear ratios, of an epicyclic mechanism, which is designed as a gear wheel stage of the synchronizing gear mechanism in the change-speed gearbox.
An advantageous embodiment of an epicyclic mechanism, in particular one which is short in the axial direction, having part-ratios for a reverse gear and at least one forward gear, which mechanism can be used as a gear wheel stage of a synchronizing gear mechanism in the change-speed gearbox according to the invention.
In the change-speed gearbox configuration as a two- or three-shaft gearbox in accordance with certain embodiments of the invention has proven advantageous.
In the change-speed gearbox according to the invention, the following advantages have essentially been achieved:
A synchronizing gear mechanism with two gear wheel stages, one of which can be designed as an epicyclic mechanism, enables the input shaft of the change-speed gearbox to be accelerated or decelerated by means of its output shaft.
The gear wheel stages of the synchronizing mechanism can balance at least the transmission ratio of the highest gear stage (e.g. between first gear and second gear). A larger spread for this transmission ratio is possible to enable several gears to be skipped by means of just one gear change operation. This means that a gear change can take place without restrictions on the choice available.
The synchronizing gear mechanism or its gear wheel stages can themselves form the transmission ratio of one or more forward gears (e.g. the lowest and the highest) and/or of the reverse gear. This considerably reduces the installation space, the weight and costs. The invention is described in greater detail below by